Sole Construction for Elastic Energy Return

ABSTRACT

In accordance with one implementation, a sole construction for energy rebound and storage includes one or more actuators located below primary pressure points of the foot. Each actuator may include a plunger element positioned above an elastic membrane. A cavity positioned below the elastic membrane may be vertically aligned with the plunger element and sized and configured to receive a portion of the membrane. The elastic membrane may be sized and configured to cradle and receive the plunger element so that when force is applied to the plunger element, the elastic membrane may stretch beneath the plunger toward the base of the cavity and store energy. A portion of this energy may be returned through the plunger element when the force is released and the elastic membrane springs back into place.

BACKGROUND

Maintaining proper alignment and stability while running are important factors in reducing the risk of injury. Many athletic shoes are designed to cushion the foot and to help absorb impact when the foot contacts the ground. However, by dampening the shock of impact using traditional means such as EVA Foam, kinetic energy is dissipated as heat rather than stored and reused by the runner. A shoe capable of storing some potential energy increases running economy and may also encourage efficient running form.

SUMMARY

Implementations described herein may be utilized to address at least one of the foregoing problems by providing a shoe sole construction that stores energy when a compressive weight is placed thereon and returns energy when the weight is taken off. In one implementation, the shoe sole construction includes a foundation layer having a cavity and an elastic membrane suspended across the cavity. The elastic membrane is configured to receive a plunger element and to protrude into the cavity when a force is applied to the plunger element. In another implementation, an air gap layer is defined between the elastic membrane and a base of the cavity.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. These and various other features and advantages will be apparent from a reading of the following Detailed Description.

BRIEF DESCRIPTIONS OF THE DRAWINGS

A further understanding of the nature and advantages of the present technology may be realized by reference to the figures, which are described in the remaining portion of the specification.

FIG. 1 illustrates a cross-sectional view of an example sole construction.

FIG. 2 illustrates a bottom perspective view of another example sole construction.

FIG. 3 illustrates a cross-sectional perspective of another example sole construction.

FIG. 4 illustrates a cross-sectional perspective of another example sole construction.

FIG. 5 illustrates an exploded top perspective view of another example sole construction.

FIG. 6 illustrates a cross-sectional perspective of another example sole construction with a pre-tensioned actuator.

FIG. 7 illustrates example operations of energy storage and rebound in a sole construction.

DETAILED DESCRIPTIONS

Recent studies have shown that running with a “natural running” form can help to reduce the frequency and severity of some common running injuries. “Natural running” refers to a form of running that a habitually barefoot runner adopts to reduce loading rates and protect the foot from excessive impact while moving quickly and efficiently. A runner practicing the natural running form strikes the ground close to the body's center of gravity with a relaxed foot, rather than over striding with an aggressively dorsiflexed ankle. Therefore, running shoes that provide a lot of cushioning in the heel can make it difficult for a runner to land with a relaxed, natural foot position because elevated heel can cause a runner to engage the foot early in the gait cycle. This often leads to over striding and increased loading rates due to relatively straight legs on impact.

Traditional walking and running shoes cushion the foot and help to absorb impact when the foot contacts the ground. However, by dampening the shock of impact using traditional means, kinetic energy is dissipated as heat and not reused by the runner. However, some of this energy can be effectively “stored and returned” to the runner if it is converted to potential energy, temporarily stored in the shoe, and returned to the runner's foot when the runner pushes off the ground. A shoe capable of storing and returning some energy in this way reduces energy conversion to heat, and consequently improves running economy. Additionally, such a shoe may also encourage more efficient running form. For instance, natural running seeks to mitigate or eliminate braking from over striding. Some runners who are learning natural running form have difficulty reducing over striding the ground. Therefore, an ideal running shoe may store and return energy and mitigate over striding by reducing interference of an efficient gait cycle. Additionally, the shoe may promote speed and efficiency by providing for one or more pre-tensioned actuators under key pressure points of a wearer's foot that improve storage and return of energy.

The implementations disclosed herein provide for one or more of such actuators, each including a cavity below an elastic membrane and a plunger positioned above the elastic membrane and vertically aligned with the cavity. The plunger stretches a portion of the membrane toward or against the base of the cavity when a runner's weight is applied to a top side (e.g., a foot-facing side) of the plunger, converting the kinetic energy of the runner to potential energy in the membrane. As the weight is removed from the plunger, the elastic membrane springs back into place, transferring the potential energy back to the runner as kinetic energy.

FIG. 1 illustrates a cross-sectional view of an example sole construction 100. The sole construction 100 includes a heel region 102, a metatarsal region 104, and a toe region 106. The heel region 102 preferably underlies or substantially underlies the entire width of a heel of a wearer's foot. The metatarsal region 104 is positioned forward or anterior to the heel region 102, and underlies or substantially underlies the metatarsal bones of a wearer's foot, both medial to lateral and posterior-to-anterior. The toe region 106 is positioned forward or anterior to the metatarsal region 104 and underlies or substantially underlies the toes of a runner's foot.

The sole construction 100 includes a foundation layer 120 including an upper surface (e.g., a foot-facing surface) that is configured to receive and cradle a wearer's foot. The sole construction 100 also includes at least one actuator element (e.g., actuator elements 108 and 110) for energy storage and rebound, which may be positioned in the heel region 102 and/or the metatarsal region 104 of the sole construction 100 below one or more primary pressure points of the wearer's foot. The implementation shown illustrates two actuator elements—a heel actuator 110 in the heel region 102 and a metatarsal actuator 108 in the metatarsal region 104. The heel actuator 110 and the metatarsal actuator 108 each include a cavity 112, an elastic membrane 114, and a plunger element 116 vertically aligned with the corresponding cavity 112. The elastic membrane 114 has one or more edges suspended above the cavity 112 and a portion nesting within the cavity 112.

The elastic membrane 114 of the heel actuator 110 is separated from a base of the cavity by an air gap layer 118. In contrast, the elastic membrane 114 of the metatarsal actuator 108 contacts a base of the associated cavity 112 across substantially the entire lower surface of the elastic membrane 114. In both such implementations, the plunger element 116 rests within the corresponding cavity 112 above the corresponding elastic membrane 114 so as to operationally engage the membrane when force is applied to the plunger element 116.

The cavity 112 (of either actuator 108 or 110) is formed in the foundation layer 120 of the sole construction 100. The cavity 112 is sized and configured to receive the plunger element 116 and a portion of the elastic membrane 114. Additionally, the elastic membrane 114 is shaped to receive and cradle the plunger element 116. In the implementation shown, the elastic membrane has a portion that nests within the cavity 112 and receives the plunger element 116.

In the actuator 110, the portion of the elastic membrane 114 nesting within the cavity 112 is separated from the base of the cavity 112 by the air gap layer 118. Upon impact, a runner's foot pushes the plunger element 116 against the elastic membrane 114, stretching the elastic membrane 114 further into the cavity 112 and toward the base of the cavity 112. When the runner's weight is removed from the plunger element 116, the elastic membrane 114 springs back into place, returning energy through to the plunger element 116 and ultimately, to the runner's foot.

In the actuator 108, a portion of the elastic membrane 114 nests snugly within the cavity 112 so that there is no air gap layer 118. In this case, the elastic membrane 114 may be adjacent to and in contact with the cavity 112 along some or substantially the entire base of the cavity 112. Upon impact, the runner's foot pushes the plunger element 116 against the elastic membrane 114, stretching the elastic membrane 114 against the foundation layer 120. When the runner's weight is removed, the elastic membrane 114 springs back into place. In the same or another implementation, the foundation layer 120 compresses under the force of the plunger element 116. When the runner's weight is removed, the foundation layer 120 decompresses.

The sole construction 100 also includes a soft upper 124 (e.g., fabric forming the top of the shoe) attached to a stability layer 122. The upper 124 may be attached to the stability layer 122 in a variety of ways such as stitching, adhesives, etc. For example, the upper 124 may be attached to the stability layer 122 by stitching around the periphery of the stability layer 122. Other attachment mechanisms may also be employed to bond the upper 124 to the stability layer 122.

The stability layer 122 may be foam (e.g., EVA), rubber, fiberboard (e.g., Strobel Board) or other flexible material. In one implementation, the stability layer 122 is a Strobel Board about 2 mm thick, which is sewn to the upper 124. Other methods of attachment may also be employed to bond the stability layer 122 to the foundation layer 120.

The stability layer 122 is adjacent to and in contact with the foundation layer 120 (e.g., on a side opposite of the upper 124). The stability layer 122 may attach to the foundation layer 120 in such a way as to effectively pre-tension the elastic membrane 114. Pre-tensioning creates a force on the elastic membrane 114 even before a runner puts pressure on the actuator 108 or 110 by applying weight. Thus, pre-tensioning ensures contact of the plunger 116 with the elastic membrane 114 before impact to provide a quick elastic response upon impact. In one implementation, pre-tensioning is accomplished by attaching the stability layer 122 to the foundation layer 120 and/or the plunger 116 in such a way as to apply a slight downward pressure to the plunger 116. For example, pre-tensioning may be accomplished by gluing the stability layer 122 to the foundation layer 120 with a last inserted. When the glue hardens, the last can be removed.

The number and position of the actuator elements (e.g., actuators 108, 110) may vary depending upon specific design criteria. However, in various implementations, the actuators may be positioned in any region (e.g., heel, metatarsal, or toe) of the sole construction 100. Such actuators may have the features of actuator 108, 110, or of other implementations described herein. The construction 100 may include more than one actuator elements in any region and/or no actuators in other regions. For example, there may be one or more actuators in the heel region 102 and no actuators in the metatarsal. Alternatively, the sole construction 100 may include one or more actuator elements in the metatarsal region 104 and no actuators in the heel region 102.

FIG. 2 illustrates a bottom perspective view of another example sole construction 200. The sole construction 200 includes a foundation layer 220, which spans the length of the shoe 200 from a heel region 202 to the tip of a toe region 206. The foundation layer 220 has a lower surface adjacent to one or more foam heel pieces comprising a heel layer 226 positioned in the heel region 202 of the sole construction 200. The heel layer 226 includes one or more traction elements (e.g., a traction element 228) that provide tread for the shoe and prevent the shoe from “slipping” against the ground when in use. The traction elements may be separate from the heel layer 226 and metatarsal layer 230, or may be texture that is formed on the heel layer 226 and/or metatarsal layer 230.

The lower surface of the foundation layer 220 also contacts one or more foam metatarsal pieces comprising a metatarsal layer 230. The metatarsal layer 230 attaches to the foundation layer 220 and includes one or more of the traction elements 228.

One or more actuator elements (e.g., an actuator 210) are positioned within the sole construction 200 above the foundation layer 220. In FIG. 2, an actuator 210 (which may be the same or similar to the actuators described with respect to FIG. 1) is positioned in the center of the heel region 202 of the sole construction 200 above the foundation layer 220, such that the actuator 210 is situated between the foundation layer 220 and a runner's foot when the sole construction 200 is in use. In the same or an alternate implementation, the sole construction 200 has one or more actuator elements in the metatarsal region 204.

FIG. 3 illustrates a cross-sectional perspective of another example sole construction 300. The sole construction 300 has an internal actuator 310, which may be positioned under a primary pressure point of a wearer's foot. For example, the internal actuator 310 may be formed in a heel portion of the shoe (below the center of a runner's heel) or in a metatarsal region of the shoe, forward of the heel portion, below the ball of the wearer's foot. In other implementations, more than one actuator 310 may be used in the same sole construction and more than one actuator may be found in the same region of the sole construction.

The sole construction 300 includes a foundation layer 320 having a top side 322 (e.g., a foot-facing side) defining a central cavity 312 therein. The actuator 310 includes an elastic membrane 314 that is adjacent to the foundation layer 320. The elastic membrane 314 has a bucket portion 340 and a rim portion 342. The rim portion 342 of the elastic membrane 314 rests above and adjacent to the foundation layer 320 and the bucket portion 340 of the elastic membrane 314 nests within the foundation layer cavity 312. In the implementation illustrated by FIG. 3, the cavity 312 and the bucket portion 340 of the elastic membrane 314 are sized and configured such that an air gap 318 is formed between a base of the bucket portion 340 and a base of the cavity 312 when the bucket portion 340 is suspended in the cavity 312. The depth of the air gap layer 318 may vary with design criteria to provide for more or less cushioning (e.g., a harder or softer ride) to accommodate the physical needs, running styles, and use preferences of different wearers. However, in one implementation, the air gap layer 318 is equal to about three millimeters in height.

In at least one implementation, the gap layer 318 is filled with a soft material, such as a gel, rubber insert, or soft material such as EVA foam.

A plunger 316 is positioned within the bucket portion 340 of the membrane 314 and vertically aligned with the cavity 312. The plunger of each of FIGS. 3-5 (i.e., elements 316, 416, and 516) has substantially flat ends (e.g., the ends substantially parallel to the base of the cavity 312) and an elliptical cross section; however, a variety of other shapes and sizes may also be employed including without limitation rectangular, cylindrical, or other non-traditional shapes.

As illustrated in FIG. 3, the plunger 316 may have a height that is greater than the depth of the bucket portion 340 of the elastic membrane 314 such that the plunger extends a distance 344 above the elastic membrane 314 when the plunger is positioned within the bucket portion 340 and no external forces are applied. In one implementation this distance 344 is about 1-3 mm.

When a runner's weight is pushed against the internal actuator 310, the plunger 316 is pushed down against the elastic membrane 314 and into the gap 318 below the elastic membrane, stretching the elastic membrane 314 toward the base of the cavity 312 and converting kinetic energy from the impact into potential energy in the actuator 310. When the weight of the runner is removed from the actuator, the elastic membrane 314 springs back to its original position, returning potential energy to the runner. In this manner, some of the runner's energy is stored and returned rather than merely absorbed and lost.

FIG. 4 illustrates a cross-sectional perspective of another example sole construction 400. The sole construction 400 has an internal actuator 410, which may be positioned under a primary pressure point of a wearer's foot. The sole construction 400 includes a foundation layer 420 having a top side 422 defining a central cavity 412 therein. The actuator 410 includes an elastic membrane 414 that has a bucket portion 440 and a rim portion 442. The rim portion 442 rests above and adjacent to the foundation layer 420 and the bucket portion 440 nests within the foundation layer cavity 412 adjacent to and in contact with substantially the entire base of the cavity 412.

A plunger 416 rests within the bucket portion 440 of the membrane 414 and is vertically aligned with the cavity 412. The plunger 416 may be made from a harder material than the foundation layer 420 such that the foundation layer 420 compresses beneath the plunger 416 when a force is applied to an upper side (i.e., foot-facing side) of the plunger 416. When such force is applied to the plunger 416, the elastic membrane 414 is stretched toward the underside (i.e., a ground-facing side) of the sole construction 400.

FIG. 5 illustrates an exploded cross-sectional perspective view of another example sole construction 500. The sole construction 500 has a foundation layer 520 with a top side 546, a bottom side 548, and a central cavity 512 defined therein. The cavity has a substantially flat base 550 and a single, curved sidewall 552 defining an oval-shaped space within the cavity; however, other cavity shapes are contemplated.

The depth of the cavity 512 may be greater than or substantially equal to a depth 568 of a bucket portion 540 of an elastic membrane 514 that nests within the cavity 512. The base 550 of the cavity 512 is shown substantially parallel to the bottom side 548 of the foundation layer 520. However, the cavity 512 may also have a base of variable depth (e.g., a sloped base). In an example implementation, the cavity 512 is about 8 mm deep on a first side (e.g., a side closer to a metatarsal region of the sole construction) and about 10 millimeters deep on a second side (e.g., a sider closer to a heel portion of the sole construction).

The foundation layer 520 of the sole construction 500 may be composed of ethylene-vinyl acetate (EVA), or other foam or soft, pliable material. In some implementations, an elastomeric viscous foam or gel may be used. The hardness may vary depending on design criteria. However, in some implementations, the hardness of the foundation layer 520 ranges from about 54 to about 60 Shore C.

The sole construction 500 has an elastic membrane 514 with a rim portion 542 and a bucket portion 540. The rim portion 542 may be positioned adjacent to the top side 546 of the foundation layer 520, and the bucket portion 540 may be nested within the cavity 512. The bucket portion 540 may be cylindrical, square, rectangular, or any one of a number of shapes sized to fit within the cavity 512. In the implementation shown, the bucket portion 540 is shaped like a “bucket” and has a base 558 and a single curved sidewall 560 defining an internal space. In an implementation where the bucket portion 540 is of a different shape, the bucket portion may have more than one sidewall defining the internal space within the elastic membrane 514. In one implementation where the cavity 512 has a sloped base, the base 558 of the elastic membrane 514 is similarly sloped so that it is substantially parallel to the base 550 of the cavity 512.

The rim portion 542 of the membrane 514 connects to the bucket portion 540 at an upper (e.g., foot-facing) edge of a sidewall of the bucket portion 540. The rim portion 542 surrounds the bucket portion 540 and extends away from the center of the bucket portion 540. In one implementation, the rim portion 542 extends away from the center of the bucket portion 540 in a direction substantially parallel to the base 550 of the cavity 512. In another implementation, the rim portion 542 of the elastic membrane 514 extends away from the center of the bucket portion 540 in a direction non-parallel to base 550 of the cavity 512.

The elastic membrane 514 may be made of any highly resilient elastic material such as rubber, synthetic rubber, DuPont Hytrel™, and highly resilient elastic foams. The elastic response of the membrane depends on its durometer and thickness. In some implementations, the thickness of the elastic membrane 514 may range between 0.5 mm or less to about 4 mm or more, including 1 mm, 2 mm, and 3 mm. In one example implementation, the elastic membrane 514 is 1.5 mm thick DuPont Hytrel™. The selection of material hardness and thickness of the elastic membrane 514 depends upon specific design criteria that may vary with the particular function of the shoe (i.e., running, walking, cross-training, etc.). However, In one example implementation, the elastic membrane 514 is about 20 Shore A.

The bucket portion 540 of the elastic membrane 514 has at least one cross section 564 that is smaller than a cross section of the cavity 512, such that some or all of the bucket portion 540 may nest within the foundation layer cavity 512. The rim portion 542 of the membrane 514 has at least one edge-to-edge cross section 566 that is greater than a cross-section of the foundation layer cavity 512 so that the rim portion 542 may rest above and adjacent to the top side 546 of the foundation layer 520 with the bucket portion 540 of the membrane suspended within the cavity 512.

In one implementation, the rim portion 542 of the elastic membrane 514 has an elliptical cross-section with a semi-major axis of about 71 mm, and the bucket portion 540 of the elastic membrane 542 has an elliptical cross-section with a semi-major axis 564 of about 51 mm. In another implementation, the rim portion 542 of the membrane 514 has an edge-to-edge cross section that is substantially equal to a lateral width of the sole construction 500. Other widths are also contemplated.

When the bucket portion 540 of the membrane 514 is nested within the foundation layer cavity 512 and the rim portion 542 of the membrane 514 rests against the top side 546 of the foundation layer 510, an air gap layer (not shown) may be defined above the base 550 of the cavity 512 and below the base 558 of the bucket portion 540 of the elastic membrane 514. Thus, the bucket portion 540 of the elastic membrane 514 may have a depth 568 that is less than a depth 554 of the cavity 512.

In another implementation, the bucket portion 540 of the elastic membrane 514 has a total depth 568 that is substantially equal to a depth 554 of the foundation layer cavity 512. In one example implementation, the bucket portion 540 of the elastic membrane 514 has a sloped base that is approximately 8 mm deep on one side and approximately 10 mm deep an opposite side.

The sole construction 500 also has a plunger piece 516 that may be positioned within the bucket portion 540 of the elastic membrane 514. In FIG. 5, the plunger portion 516 is circular or oval shaped. However, a variety of other shapes are contemplated. A variety of materials may be used for the plunger 516 including without limitation EVA foam, PVA foam, PoronXrd, and combinations thereof. In one example implementation, the plunger 516 is a solid piece of ethylene-vinyl acetate.

In an implementation where there is no air gap between the base 558 of the elastic membrane 514 and the base 550 of the cavity 512, the plunger 516 may be constructed of a denser material than the foundation layer 520 so that the foundation layer 520 may compress beneath the plunger 516 when a force is applied to the upper-side (e.g., a foot-facing side) of the plunger 516. In some implementations, the plunger 516 has a hardness between about 65 and about 70 Shore C, while the foundation layer has a hardness of between about 54 and about 60 Shore C. In another implementation, the plunger 516 has a hardness of about 50 Shore C.

Dimensions of the plunger may vary; however, in one implementation, the plunger piece 516 has a height 572 that is greater than the depth 568 of the bucket portion 540 of the elastic membrane 514. In another implementation, the plunger piece 516 has a height 572 that is substantially equal to the depth 568 of the bucket portion 540 of the elastic membrane 514. Thus, a portion of the plunger piece 516 may protrude above the rim portion 542 of the elastic membrane 514 when the plunger piece 516 is positioned within the membrane bucket portion 540 and when the elastic membrane 514 is not stretched under an external force. In some implementations, the plunger 516 has a depth 572 that is about 1-3 mm greater than the depth 554 of the cavity 512 and/or the depth 568 of the bucket portion 540 of the elastic membrane 514. In one example implementation, the plunger 516 has a depth 572 of approximately 11 mm and an elliptical cross section with a semi-major axis 574 of approximately 29 mm. In the same or another implementation, the cavity 512 and/or the bucket portion 540 of the elastic membrane 514 have depths ranging from between about 8 mm and about 10 mm.

The sole construction 500 may also have a stability layer 577 positioned above the plunger piece 516 to effectively seal the plunger piece 516 into the bucket portion 540 of the elastic membrane 514. In one implementation, the stability layer 577 is attached (e.g., glued) to the foundation layer 520 so that the stability layer 577 applies a force to plunger element 516 and pre-tensions the elastic membrane 514. This force may cause the plunger element 516 to compress the foundation layer 520 below the plunger 516 or to stretch the elastic membrane 514 into a gap (e.g., an air gap) between the membrane 514 and the foundation layer 520.

In an implementation where the elastic membrane 514 is pre-tensioned within the sole construction 500, the foundation layer 520 beneath the plunger 516 (e.g., the base of the cavity 512) contacts the base of the bucket portion 540 of the elastic membrane 514. In another implementation, the foundation layer 520 beneath the plunger 516 is compressed by the plunger 516 such that the uppermost surface of the plunger 516 is substantially even in height with the rim portion 542 of the elastic membrane 540. In another implementation, pre-tensioning of the elastic membrane 514 stretches the bucket portion 540 of the elastic membrane 514 into an air gap (not shown) so that the uppermost surface of the plunger 516 is substantially even in height with the rim portion 542 of the elastic membrane 514.

FIG. 6 illustrates a cross-sectional perspective of another example sole construction 600 with a pre-tensioned actuator. The sole construction 600 has a foundation layer 620 having a top side 622 with a cavity 612 defined therein. An elastic membrane 614 is suspended across the cavity 612 and has a rim portion 642 above and adjacent to the top side 622 of the foundation layer 620 and a bucket portion 640 suspended within the cavity 612. A plunger 616 is above the elastic membrane 614 and rests within the bucket portion 640 of the elastic membrane 614. The plunger 616 has a height that is greater than the depth of the bucket portion 640 of the elastic membrane when the membrane is not pre-tensioned. However, the bucket portion 640 of the elastic membrane illustrated is stretched under a force of plunger 616. The force of the plunger 616 is applied to a top side 674 of the plunger by a lining layer 672. This “pre-tensioning” of the elastic membrane ensures contact between a base of the plunger 616 and a base of the bucket 640 of the elastic membrane, and effectively loads the elastic membrane with potential energy to provide for a quick elastic response upon impact. In the implementation shown in FIG. 6, the foundation layer 620 below the plunger 616 is compressed and thus has a decreased thickness as compared to FIG. 4 (where the membrane 414 is not pre-tensioned).

FIG. 7 illustrates example operations of energy storage and rebound in a sole construction. A force application operation 705 applies a force to a plunger element to stretch an elastic membrane toward a base of a cavity opposite the elastic membrane. In one implementation, the plunger directly contacts the elastic membrane when the force is applied. The force may be, for example, the weight of a runner applied to the sole construction upon impact with the ground.

In one implementation, the cavity is formed in a foundation layer, and the elastic membrane is suspended across the cavity by a rim portion of the elastic membrane that rests against the upper side of the foundation layer above the base of the cavity. In the same or an alternate implementation, the elastic membrane has a “bucket” portion that is suspended within the cavity. In the same or an alternate implementation, the bucket portion has a base that is separated from the base of the cavity by an air gap layer.

The bucket portion of the elastic membrane is sized and shaped to receive and cradle a plunger element. In one implementation, the plunger element has a height that exceeds the depth of the bucket portion of the elastic membrane when no external forces are applied (e.g., when the elastic membrane is not pre-tensioned). In one implementation, the elastic membrane is pre-tensioned by the plunger to provide for a quick elastic response upon impact. When force is applied to a top side of the plunger element, the elastic membrane is stretched below the plunger element and toward the base of the cavity. In one such implementation, such pre-tensioning stretches the elastic membrane into an air gap layer within the cavity. In another implementation, such pre-tensioning compresses the foundation layer.

At removal operation 710, the force is removed from the plunger element, which allows the stretched elastic membrane to spring back to an original position. The removal operation 710 coverts potential energy stored in the shoe to kinetic energy that is returned to the runner through the plunger. This process (705-710) may be repeated with each “step” of a runner wearing a shoe with the sole construction. Force is applied, energy is stored in the elastic membrane, and then the stored energy is returned to the runner.

It should be understood that operations referred to in the implementations disclosed herein may be performed in any order, adding and omitting as desired, unless explicitly claimed otherwise or a specific order is inherently necessitated by the claim language. The above specification, examples, and data provide a complete description of the structure and use of exemplary implementations of the invention. Since many implementations of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. Furthermore, structural features of the different implementations may be combined in yet another implementations without departing from the recited claims. 

What is claimed is:
 1. A sole construction comprising: a foundation layer with a cavity defined therein; an elastic membrane suspended across the cavity; and a plunger element adjacent to the elastic membrane opposite the cavity, and vertically aligned with the cavity to stretch the elastic membrane under force toward a base of the cavity.
 2. The sole construction of claim 1, wherein an air gap is defined between a base of the cavity and the elastic membrane.
 3. The sole construction of claim 1, wherein the elastic membrane has a rim portion in contact with the foundation layer and a bucket portion suspended within the cavity when no external force is applied to the sole construction.
 4. The sole construction of claim 3, wherein the bucket portion of the elastic membrane is configured to receive a portion of the plunger.
 5. The sole construction of claim 3, wherein a base of the plunger rests within the cavity.
 6. The sole construction of claim 1, wherein the elastic membrane is pre-tensioned by a force applied to the plunger.
 7. The sole construction of claim 6, wherein stretching the elastic membrane compresses the foundation layer.
 8. An actuator comprising: an elastic membrane having a portion positioned within a cavity; and a plunger element adjacent to the elastic membrane opposite the cavity, and vertically aligned with the cavity to stretch the elastic membrane under force toward a base of the cavity under.
 9. The actuator of claim 8, wherein the elastic membrane has a rim portion in contact with the foundation layer and a bucket portion suspended within the cavity when no external force is applied to the actuator.
 10. The actuator of claim 8, wherein the bucket portion of the elastic membrane is configured to receive a portion of the plunger.
 11. The actuator of claim 8, wherein a base of the plunger rests within the cavity.
 12. The actuator of claim 8, wherein the elastic membrane is pre-tensioned.
 13. The actuator of claim 12, wherein stretching the elastic membrane compresses the foundation layer.
 14. A method of providing energy return to a user comprising: applying a force to a plunger to stretch an elastic membrane toward a base of a cavity opposite the elastic membrane; and removing the force to allow the elastic membrane to return to an original position and return energy through the plunger to the user.
 15. The method of claim 14, wherein applying the force to the plunger stretches the elastic membrane into an air gap defined between the base of the cavity and the elastic membrane.
 16. The method of claim 14, wherein the elastic membrane has a rim portion in contact with the foundation layer and a bucket portion suspended within the cavity when no external force is applied to the plunger.
 17. The method 16, wherein the bucket portion of the elastic membrane is configured to receive a portion of the plunger.
 18. The method of claim 16, wherein a base of the plunger rests within the cavity.
 19. The method claim 14, wherein the elastic membrane is pre-tensioned.
 20. The method claim 19, wherein stretching the elastic membrane compresses the foundation layer. 